It is easier grooving the rotor‘ s outside on the the car. Start the engine, out a gear. Let it run and groove. 100% save. Only the inner part I Groove the way you do it.
Great method of removing rust and getting a good surface on the rotor. I would highly recommend using a dial indicator before and after to measure the run out and verify that the resurfacing eliminated any warped areas.
This method isn't for "truing" the rotor, just re-surfacing and deglazing it. Many times, if the pedal wasn't pulsing, and you just need new pads, you can deglaze the rotor and get much much more life out of them.
These methods would be much improved by using the power tools stationary with the disk revolving on a lazy susan type of contraption. It would not take much. A few angle irons bolted together to create a stand with a hole that the handle hold could be used to attach the tool into. Then just give the lazy susan with the brake rotor on it a little spin. More even, less work and less of chance of nicking the edge and surface of the rotor.
I've always just replaced the rotors with the pads. Rotors used to be cheap for my old cars. My truck is a little different. Rotors are a bit pricey. So this is the plan to refresh the ones on it that still have 60% pads after 3 years and 40 000 km. But the brake kits are cheap enough that once the pads are toast too, she's getting all new gear... might even do the calipers and hoses.
That factory groove is cut to the 2/32" so when you no longer see it through an opening in the caliper, time to replace the brake pads. For the ridges on the rotor, I just bought 2 painted rear rotors for $120 because this will be cheaper than spending 2 hours removing rust. Although, mine had way more rust than just a couple of ridges.
